1. Field of the Invention
The present invention relates to a technology for monitoring the line quality of communications lines.
2. Description of the Related Art
Synchronous Digital Hierarchy/Synchronous Optical Network (SDH/SONET) is one of several standards of optical transmission technologies, which enable high-speed line transmissions through combining and multiplexing slower lines hierarchically in order to increase transmission speeds.
In such a SDH/SONET network, remote management of the line quality of the entire network by a communications line monitoring apparatus can be realized by activating the performance monitoring function of a transmission apparatus established in a network.
Transmission data, alternatively referred to as communication frames, transmitted through the SDH/SONET network, has an overhead unit to which an area for storing the parity operation result of a communication frame transmitted prior is assigned.
First, parity operation is performed on communication frames to be transmitted in a transmission side apparatus, and result of the previously performed parity operation is written in a parity operation result storing area in the communication frames to be transmitted. The relay apparatus, which receives these communication frames, performs the same parity operation as above on the communication frame received first, compares a value of the operation result with a value of the parity operation result written in the communication frame received second, and determines the presence or absence of a failure of the line. The relay apparatus repeatedly performs the process on each of the communication frames. While the performance monitor function of the receiver side relay apparatus is activated, the receiver side relay apparatus counts the number of errors determined in units of a prescribed time such as every 15 minutes or every day. The relay apparatus autonomously transmits a notification message to the communications line monitoring apparatus at a prescribed timing such as every 15 minutes or at noon.
Note that a form in which wrong line switching does not occur in the receiver apparatus by continuing the warning output when an abnormal state continues in a section in a relayed transmission path is also disclosed (see Japanese Patent Application Publication No. 03-007439).
In recent years, the processing capacity of relay apparatuses has increased, and the capacity of a signal transmitted in one optical fiber has been dramatically increased as well. Meanwhile, the smallest unit of a path is required to remain the same as the existing lines from the market since there is a problem regarding removing the existing lines. The currently manufactured apparatuses for establishing networks have the smallest unit of path being the same as the existing ones so as to meet the requirements from the market.
Since the smallest unit of a path does not change while transmission quantity is increasing, the number of paths that one relay apparatus comprise is consequently increased. This means the increase in the number of paths per one relay apparatus monitored by a communications line monitoring apparatus results in an adverse effect such as a drastic increase in the number of processes of a communications line monitoring apparatus performing central management of each relay apparatus in the network. At present, the communications line monitoring apparatus performs various processes such as collective setting of communications lines to each relay apparatus, time management of each relay apparatus (a process for synchronizing the time of the communications line monitoring apparatus with the time of the relay apparatuses), backup of database in each relay apparatus, as well as a process for notification messages autonomously transmitted from each of the relay apparatuses and line quality management of each path based on the notification messages. Therefore, the communications line monitoring apparatus performs priority control of processes so that an administrator can perform satisfactory processes.
However, even if the priority control is performed, there may be a case that the processes are not performed sufficiently such as a case of communications line failure such that a large quantity of notification messages is transmitted from relay apparatuses. There is a plurality of types of units in which a relay apparatus checks the quality of communications lines such as units of path or units of section. If one relay apparatus detects communication failure, the following other relay apparatus detects the same communication failure, and therefore, in a case that the above communication failure occurs, the total number of notification messages transmitted from each relay apparatus to the communications line monitoring apparatus increases greatly. Consequently, congestion may be caused in the transmission paths (control lines) of notification messages, or loading of processes in the communications line monitoring apparatus may increase drastically. Because this would cause delay in execution of highly prioritized processes, it is considered as problem. In order to perform highly prioritized processes on a priority basis, generally a performance monitor function in the relay apparatuses is not activated and line quality notification messages are not transmitted from relay apparatuses to the communications line monitoring apparatus. The line quality check of a line is performed only when complaints about the line quality are received from clients using a line by activating the performance monitor function of the applicable relay apparatus.